A datagram channel is created by invoking one of the open methods
of this class. It is not possible to create a channel for an arbitrary,
pre-existing datagram socket. A newly-created datagram channel is open but not
connected. A datagram channel need not be connected in order for the send and receive methods to be used. A datagram channel may be
connected, by invoking its connect method, in order to
avoid the overhead of the security checks are otherwise performed as part of
every send and receive operation. A datagram channel must be connected in
order to use the read and write methods, since those methods do not
accept or return socket addresses.

Once connected, a datagram channel remains connected until it is
disconnected or closed. Whether or not a datagram channel is connected may
be determined by invoking its isConnected method.

Socket options are configured using the setOption method. A datagram channel to an Internet Protocol socket supports
the following options:

Datagram channels are safe for use by multiple concurrent threads. They
support concurrent reading and writing, though at most one thread may be
reading and at most one thread may be writing at any given time.

The ProtocolFamily of the channel's socket
is platform (and possibly configuration) dependent and therefore unspecified.
The open allows the protocol family to be
selected when opening a datagram channel, and should be used to open
datagram channels that are intended for Internet Protocol multicasting.

open

The family parameter is used to specify the ProtocolFamily. If the datagram channel is to be used for IP multicasting
then this should correspond to the address type of the multicast groups
that this channel will join.

bind

This method is used to establish an association between the socket and
a local address. Once an association is established then the socket remains
bound until the channel is closed. If the local parameter has the
value null then the socket will be bound to an address that is
assigned automatically.

connect

The channel's socket is configured so that it only receives
datagrams from, and sends datagrams to, the given remote peer
address. Once connected, datagrams may not be received from or sent to
any other address. A datagram socket remains connected until it is
explicitly disconnected or until it is closed.

This method performs exactly the same security checks as the connect method of the DatagramSocket class. That is, if a security manager has been
installed then this method verifies that its checkAccept and checkConnect methods permit
datagrams to be received from and sent to, respectively, the given
remote address.

This method may be invoked at any time. It will not have any effect
on read or write operations that are already in progress at the moment
that it is invoked. If this channel's socket is not bound then this method
will first cause the socket to be bound to an address that is assigned
automatically, as if invoking the bind method with a
parameter of null.

receive

If a datagram is immediately available, or if this channel is in
blocking mode and one eventually becomes available, then the datagram is
copied into the given byte buffer and its source address is returned.
If this channel is in non-blocking mode and a datagram is not
immediately available then this method immediately returns
null.

The datagram is transferred into the given byte buffer starting at
its current position, as if by a regular read operation. If there
are fewer bytes remaining in the buffer than are required to hold the
datagram then the remainder of the datagram is silently discarded.

This method performs exactly the same security checks as the receive method of the DatagramSocket class. That is, if the socket is not connected
to a specific remote address and a security manager has been installed
then for each datagram received this method verifies that the source's
address and port number are permitted by the security manager's checkAccept method. The overhead
of this security check can be avoided by first connecting the socket via
the connect method.

This method may be invoked at any time. If another thread has
already initiated a read operation upon this channel, however, then an
invocation of this method will block until the first operation is
complete. If this channel's socket is not bound then this method will
first cause the socket to be bound to an address that is assigned
automatically, as if invoking the bind method with a
parameter of null.

Parameters:

dst - The buffer into which the datagram is to be transferred

Returns:

The datagram's source address,
or null if this channel is in non-blocking mode
and no datagram was immediately available

send

If this channel is in non-blocking mode and there is sufficient room
in the underlying output buffer, or if this channel is in blocking mode
and sufficient room becomes available, then the remaining bytes in the
given buffer are transmitted as a single datagram to the given target
address.

The datagram is transferred from the byte buffer as if by a regular
write operation.

This method performs exactly the same security checks as the send method of the DatagramSocket class. That is, if the socket is not connected
to a specific remote address and a security manager has been installed
then for each datagram sent this method verifies that the target address
and port number are permitted by the security manager's checkConnect method. The
overhead of this security check can be avoided by first connecting the
socket via the connect method.

This method may be invoked at any time. If another thread has
already initiated a write operation upon this channel, however, then an
invocation of this method will block until the first operation is
complete. If this channel's socket is not bound then this method will
first cause the socket to be bound to an address that is assigned
automatically, as if by invoking the bind method with a
parameter of null.

Parameters:

src - The buffer containing the datagram to be sent

target - The address to which the datagram is to be sent

Returns:

The number of bytes sent, which will be either the number
of bytes that were remaining in the source buffer when this
method was invoked or, if this channel is non-blocking, may be
zero if there was insufficient room for the datagram in the
underlying output buffer

read

This method may only be invoked if this channel's socket is
connected, and it only accepts datagrams from the socket's peer. If
there are more bytes in the datagram than remain in the given buffer
then the remainder of the datagram is silently discarded. Otherwise
this method behaves exactly as specified in the ReadableByteChannel interface.

read

This method may only be invoked if this channel's socket is
connected, and it only accepts datagrams from the socket's peer. If
there are more bytes in the datagram than remain in the given buffers
then the remainder of the datagram is silently discarded. Otherwise
this method behaves exactly as specified in the ScatteringByteChannel interface.

read

This method may only be invoked if this channel's socket is
connected, and it only accepts datagrams from the socket's peer. If
there are more bytes in the datagram than remain in the given buffers
then the remainder of the datagram is silently discarded. Otherwise
this method behaves exactly as specified in the ScatteringByteChannel interface.

write

This method may only be invoked if this channel's socket is
connected, in which case it sends datagrams directly to the socket's
peer. Otherwise it behaves exactly as specified in the WritableByteChannel interface.

write

This method may only be invoked if this channel's socket is
connected, in which case it sends datagrams directly to the socket's
peer. Otherwise it behaves exactly as specified in the GatheringByteChannel interface.

offset - The offset within the buffer array of the first buffer from
which bytes are to be retrieved; must be non-negative and no
larger than srcs.length

length - The maximum number of buffers to be accessed; must be
non-negative and no larger than
srcs.length - offset

Returns:

The number of bytes sent, which will be either the number
of bytes that were remaining in the source buffer when this
method was invoked or, if this channel is non-blocking, may be
zero if there was insufficient room for the datagram in the
underlying output buffer

write

This method may only be invoked if this channel's socket is
connected, in which case it sends datagrams directly to the socket's
peer. Otherwise it behaves exactly as specified in the GatheringByteChannel interface.

The number of bytes sent, which will be either the number
of bytes that were remaining in the source buffer when this
method was invoked or, if this channel is non-blocking, may be
zero if there was insufficient room for the datagram in the
underlying output buffer

getLocalAddress

Where the channel is bound to an Internet Protocol
socket address then the return value from this method is of type InetSocketAddress.

If there is a security manager set, its checkConnect method is
called with the local address and -1 as its arguments to see
if the operation is allowed. If the operation is not allowed,
a SocketAddress representing the
loopback address and the
local port of the channel's socket is returned.